A pdf to mp4 conversion is somewhat odd type of file conversion, although not impossible. Considering MP4 format is just a container, it might contain pictures which can be copies of individual pages from the PDF document. This is typically used by users that need to view a PDF document on devices that only support MP4 files, for example some smart TVs.
You can achieve this either manually with some video editing tools in which you create .mp4 file from pictures you make from your PDF document, or perhaps someone made utility designed for this purpose.